Means and method for achieving an optimum operating condition for an alkylation unit

ABSTRACT

A system controls the temperature at which olefins are contacted with an isoparaffin in the presence of acid in an alkylation unit, and the flow rate of acid entering or leaving the alkylation unit. The system senses conditions of a hydrocarbon product, which results when the acid is removed from a hydrocarbon-acid mixture after the contacting process, the contact temperature, a condition relating to the contact acid and the olefin composition. The system includes a source providing signals corresponding to the economic values related to the octane rating of alkylate, which is produced from the hydrocarbon product, to the acid consumption and to the cost of controlling the contact temperature. The system uses the signals corresponding to the sensed conditions and the economic value signals to develop control signals for apparatus that control the contact temperature and the acid flow rate.

United States Patent 11 1 1111 3,778,603

Sweeney, Jr. Dec. 11, 1973 MEANS AND METHOD FOR ACHIEVING PrimaryExaminerEugene G. Botz AN OPTIMUM OPERATING CONDITION Art0rneyThomasWhaley et 3].

FOR AN ALKYLATION UNIT [75] Inventor: Donald E. Sweeney, Jr., Beaumont,

' Tex.

[57] ABSTRACT A system controls the temperature at which olefins arecontacted with an isoparaffin in the presence of acid [73] Asmgnee:Texaco Inc-r New York in an alkylation unit, and the flow rate of acidentering [22] Filed; May 26, 1972 or leaving the alkylation unit. Thesystem senses conditions of a hydrocarbon product, which results when[21] Appl- NOJ 257,408 the acid is removed from a hydrocarbon-acidmixture after the contacting process, the contact temperature,

5 Cl 235/15L12, 203/133 ZQS/DIG. 1 a condition relating to the contactacid and the olefin 260/68359 235/1501 composition. The system includesa source providing 5 Int G0 15/46, 606g 7/58, G051) 13/02 signalscorresponding to the economic values related [58] Field of Search235/151.12, 150.1 to the Octane rating of alkylate, which is Producedfrom the hydrocarbon product, to the acid consumption and to the cost ofcontrolling the contact temperature. The system uses the signalscorresponding to the sensed conditions and the economic value signals[56] References Cited UNITED STATES PATENTS to develop control signalsfor apparatus that control 3,002,818 10/1961 Berger 235/15112 theContact temparature and the and flow rate 12 Claims, 6 Drawing FiguresALKYLATE SIGNAL M EA NS 37 CHROM- COOLANT ATOGRAPHY MEANS 34 SETTLERCONTROL 12 COM P7L{TER NTACTOR 4 304 NTRO FRESH ACID FRC 300 DlSCHARGEATOGR PH ACID EANS 36 SOURCE OF D.C. V0 LTAGES PATENTED HEB I l i973EEEEEOQ i 9 MEANS AND METHOD FOR ACHIEVING AN OPTIMUM OPERATINGCONDITION FOR AN ALKYLATION UNIT BACKGROUND OF THE INVENTION 1. Field ofthe Invention The present invention relates to control systems ingeneral and, more particularly, to a control system for controlling analkylation unit to achieve an optimum condition.

2. Description of the Prior Art Heretofore control systems such asdisclosed in U.S. applications Ser. Nos. 169,385 and 169,443, now U.S.Pat. No. 3,729,629, both of which are assigned to Texaco lnc., assigneeof the present invention, controlled an alkylation unit to achieve amore rapid response in controlling the acid strength to changes in thecomposition of the olefin and isoparaffin stream or in the processitself. Neither of the disclosed systems dealt with the problem ofachieving an optimum control system for an alkylation unit. Nor is itobvious from the information disclosed in those applications how onewould go about achieving an optimum operating condition for analkylation unit.

Another control system disclosed in US. application Ser. No. 118,374,now US. Pat. No. 3,720,730, Inc., assignee of the present invention,concerns the problem of reducing the stabilizing time required for analkylation unit where there are several acid settlers being usedsimultaneously. The last mentioned application does not concern itselfwith achieving an optimum condition for an alkylation unit.

SUMMARY OF THE INVENTION A system controls an alkylation unit in such amanner so as to achieve an optimum operating condition for thealkylation unit. The alkylation unit includes a contactor wherein anolefin-isoparaffin mixture is contacted with acid at a temperaturecontrolled by a utility. The contactor provides an acid-hydrocarbonmixture to a settler which separates the acid from ahydrocarbon product.The hydrocarbon product includes alkylate. -A portion of the separatedacid is discharged from the alkylation unit while the remainingseparated acid is fed back to the contactor. Fresh acid is added to thefeedback acid to replace the discharged acid. The contactingtemperature, the operating rate of the utility, a condition related tothe strength of the contact acid and a condition of the alkylate aresensed by sensing circuits which provide corresponding signals. A signalsource provides signals corresponding to economic values related to theoctane rating of the alkylate, to the acid consumption by the alkylationunit and to the utility. A control network provides control signals inaccordance with the signals from the sensing circuits and from thesignal source. At least one control signal is used to control thecontacting temperature while another control signal is used to controleither the acid entering the alkylation unit or the acid beingdischarged from the alkylation unit.

The objects and advantages of the invention will appear hereinafter fromconsideration of the detailed description which follows, taken togetherwith the accompanying drawings wherein one embodiment of the inventionis illustrated by way of example. It is to be expressly understood,however, that the drawings are for illustration purposes only and arenot to be construed as defining the limits of the invention.

DESCRIPTION OF THE DRAWINGS FIG. 1 includes a simplified block diagramof a con- I DESCRIPTION OF THE INVENTION Referring to FIG. I, there isshown a portion of an alkylation unit in which an olefin is reacted withisoparaffin at a predetermined temperature and in the presence of acatalyst, such as sulfuric acid and which is hereinafter referred to asthe reaction acid, to form a higher molecular weight isoparaffin. Forpurpose of explanation, the acid in the following description shall besulfuric acid. The, olefin may be butylenes, propylene or a mixture ofbutylenes and propylene, while the isoparaffin may be isobutane. Thecontrol system shown in FIG. 1 controls the reaction temperature and theflow rate of discharge acid leaving the alkylation unit to achieve anoptimum operating condition. The flow rate of fresh acid entering thealkylation unit may be controlled instead of the discharge acid. I

The oletins and isoparaffin enter a contactor 4 by wayof a line 6, wherethe olefins and isoparaftin are contacted with the reaction acidentering by way of a line 7. Contactor 4 provides an acid-hydrocarbonmix by way of a line 8 to an acid settler 12. The reaction temperatureis controlled by coolant passing through a coil 13 inside contactor 4.Settler 12 separates the hydrocarbon product from the reaction acid. Thehydrocarbon product, which includes alkylate, is removed through a line14 for further processing while the reaction acid is removed by way of aline 16. Acid settler 12 may be the only acid settler in the unit or itmay be the last acid settler of a group of acid settlers. Fresh acidenters line 16 by way of a line 17 as needed to maintain a desiredreaction acid strength. A pump 20 pumps the reaction acid from line 16into line 7. A portion of the reaction acid in line 7 is discharged byway of a line 21. The discharge acid may be provided to anotheralkylation unit or disposed of.

In order that an optimum operating condition, such as the contactortemperature, be determined, it is necessary that a differential earningsE be determined in accordance with the following equation:

where A Q is the differential octane rating of the alkylate productresulting from a change in the contactors temperature, W is theincremental worth of a unit change in octane rating, A A is thedifferential acid consumption resulting from a change in the contactorstemperature, W is the cost of acid, A U, through A U, are differentialutilities requirements for utilities 1 through n resulting from a changein the contactors temperature, W through W are the cost of utilities 1through n. For purpose of illustration, one utility will be hereinafterdescribed and that utility concerns the 3 coolant passing through coil13. Therefore, equation 1 may be written as:

The terms W W and W are predetermined economic values for the octanerating, the acid and the utility, while A Q, A A and A U are determinedas follows:

3- A Q GC GB where G is a calculated quantity based on sensed conditionssuch as the sensed contactor temperature T; and the ratio P of propyleneto olefins in line 6, while 6 is the same type of quantity calculated ata contactor temperature other than the sensed temperature and for theexisting propylene to olefin ratio. The quantities G and G may becalculated from the following equations 4 through 6 by substituting G orC for G G or 6 t/ 1.)] GU where T may be the sensed temperature T, or acalculated temperature T,, T, is a first reference temperature which, byway of example, may be 45F and T, is a higher reference temperaturewhich for purposes of i1- lustration may be 55F. The selection of whichequation to use in calculating G and 6, depends on the ratio of thepropylene to olefins in line 6. When the actual ratio R, is below apredetermined ratio P equation 4 is used. When the ratio P, is aboveanother predetermined ratio P equation 5 is used. When the ratio P, isgreater than P but less than F equation 6 is used. By way of example, Pmay be 0.35 while P may be 0.60. The following Table I shows values fora, through a, and b, through b,, which have been determined for anexisting alkylation unit.

TABLE I a, 2.l7592 b, 4.7 a, -14.668939 b, -s.sss239 a, 37.63l287 b, 0.0a. -29.92o927 b -9.4274o9 0.0 b, 0.0

The differential acid consumption A A is determined from equation 7.

7. A A =Ag 1 where A 'is the existing acid consumption, F, is a quantitycalculated at the existing contactor temperature and the existingpropylene ratio, while F is a similar quantity calculated at any othercontactor temperature and the existing propylene ratio. The existingacid consumption A, is obtained from the following equation 8:

RUB/RX where R is a sensed discharge acid flow rate, R is the alkylaterate and a is a constant to convert the discharge acid rate from barrelsper hour to tons per hour and by way of example may be 0.3213.

The calculated acid consumption A for a current calculation step is:

9. A6 Ag A Equation 9 may be rewritten as:

10 AC A5 '1' (5 1 which simplifies to 11. AC=AB (FE/Fe) Using equation8, which is applicable for A by substituting R for R the calculateddischarge acid flow rate R may be solved to yield equation 12.

nc m; a/ e) The alkylate product rate R is obtained from equation 13.

13. R VKRC where V is the volume fraction of alkylate product in thecrude alkylate stream, R is the crude alkylate flow rate. The quantitiesF B and F are obtained from equations 14, 15 and 16 by substituting thesensed temperature T, for T when the quantity F, is desired and thecalculate temperature T when the quantity F is desired.

14. F 1.0 c [(TT1)/100] C2[(T T1)/1O0]2 3[( l)/ 4[( 1)/ 5[( 1)/ 15. F=1.0 d [(TT,)/100] .d [(T-T;)/100] :r[( z)/100l 4[( z)/100] s[( :)/l00]FL-U [(PU PB)/(PU PL)] 1. [(PBPL)/' u 1.)] u 1 Equation 14 is used whenthe ratio P is less than the reference ratio P equation 15 is used whenthe ratio P, is greater than the reference ratio P and equation 16 isused when the ratio P is equal to or greater than the ratio P but lessthan or equal to ratio P Table 1] relates specific values for the termsc, through c, and d through d, in equations 14 and 15 for a particularalkylation unit.

TABLE 11 TERM VALUE TERM VALUE c -2.901263 4, 7.7l9104 0, 1.3076425 d,29.247134 0, 0.0 d; 0.0

0 0.0 4 -2l1.32222 c, 0.0 d. 0.0

culated temperature T, is substituted for T when the quantity H isdesired.

The following table contains specific values for the co-efficients ethrough c in equation 18 as determined for a particular alkylation unit.

TABLE 111 e, 6.005l3l 2-, 0.0011537 e 0.0 e, 0.26122 e. 0.0 e 0.0 e,-0.002753 e, 5.82 e 1.5 e 0.0 e 0.l538 e 0.0 e. 2.1067 c 0.0 e 0.0 e.0.0

The calculated utility rate R which in this case is the coolant flowrate, is determined from equation 19.

uc RUB U )(RK) where R is the sensed utility rate which happens, in thiscase, to be the sensed coolant flow rate.

The control system as hereinafter described calculates the differentialearnings for selected temperatures and when the differential earningsdecreased or an operating parameter exceeds a constraint limit, thecontrols system returns to the next previous selected temperature andchanges the contactor 4 temperature and the discharge acid flow rateaccordingly to obtain an optimum operating condition. The selectedtemperatures are determined in accordance with the following equation20.

l TB l( j)/ l( TM) where J is any positive integer, j is a selectedinteger from zero to J and A T is the maximum allowable change in thecontactor temperature.

Referring to FIGS. 1 and 2, programmer 27 receives a voltage V, from asource 28 of direct current voltages. An operator initiates theoptimizing control by closing switch 26 to apply voltage V to clockmeans 34. Switch 26 may be an on-off toggle switch. Clock means 34provides pulses IE at a repetition rate which corresponds to a periodicinitiation of an operation sequence. Pulse E resets chromatograph means36 and 37.

Chromatograph means 36 samples the olefin and isoparaffin stream in line6 and provides a pulse signal E to programmer 27 and a continuous signalE to P signal means 41. The peaks of signal 13, correspond to theconcentrations of the constituents of the olefins and isoparaffin streamin line 6.

Reset pulse E, is also applied to control pulse circuit 43 to reset acounter 44. With counter 44 at a zero count, a logic decoder 45 providesa high level direct current voltage to an AND gate 48 which alsoreceives signals E, from chromatograph means 36. The pulses pass throughthe enabled AND gate 48 and are counted by counter 44. Logic, decoder 45decodes the count in counter 44 to provide a plurality of outputs to aplurality of one shot multivibrators 52. Each multivibrator ofmultivibrators 52 is triggered by a different output from logic decoder45 and provides a control pulse coinciding with a different peak ofsignal 13,. Multivibrators 52 provide control pulses B, through E whichis shown as being on one line. Pulse E triggers another one shotmultivibrator 53 which provides a time delay pulse whose width is ofsufficient duration to allow calculations to be made by P signal means41. The delay pulse from multivibrator 53 triggers another multivibrator54 which provides control pulse E Programmer 27 provides control pulsesE through E,; to P signal means 41.

Signal means 41 provides a signal E corresponding to the ratio P, ofpropylene in line 6 to the oletins in line 6. Signal means 41 is similarto the olefin signal means disclosed and described in a U.S. applicationSer. No. 228,826 filed on Feb. 24, 1972. The aforementioned applicationis assigned to Texaco Inc., assignee of the present invention. In theaforementioned application, signal E corresponds to signal E in thepresent application while control pulses E, through E have the samefunction as described in the aforementioned application. Signal means 41also receives scaling voltages V, through V from source 28, whichcorresponds to the scaling voltages V through V in the aforementionedapplication. The output from the olefin signal means represents a ratiohaving a range from 0 to 100, P signal means 41 is modified, so that E,corresponds to a range of O to l, in a manner which is obvious to oneskilled in the art.

Chromatograph means 37 samples the hydrocarbon product stream in line 14and provides a pulse signal E to a programmer 27 and a signal E, toalkylate signal means 56 receiving direct current voltages V through Vfrom source 28 and control pulses E through E from programmer 27. Thepeaks of signal E correspond to the concentrations of differentconstituents of the stream in line 14. Each pulse in pulse signal Ecoincides with a different peak of signal E-,. Alkylate signal means 56is controlled by pulse signals E through E to provide a signal Ecorresponding to the concentration of alkylate in the stream in line 14.Alkylate signal means 56 is similar to the signal means desribed in U.S.application Ser. No. 169,443 filed Aug. 5, 1971 and assigned to Texacothe present invention, except that signal 13, corresponds to anormalized concentration constituent with six or more carbon atoms.Alkylate-signal means 56 samples and holds the peaks of signal Ecorresponding to the concentrations of propane, isobutane, n-butane,pentanes, and all compound with six or more carbon atoms. Alkylatesignal means 56 normalizes the determined concentration of compoundswith six or more carbon atoms by summing all of the sampled peaks ofsignal E and dividing into the sampled peak of signal 1 pounds of six ormore carbon atoms, to provide signal E5.

Pulse signal E is applied to a control pulse circuit 43A, which issimilar in operation to control pulse circuit 43, which provides controlpulses B, through E Pulse E is also applied to a delay one shotmultivibrator 60 which provides a delay pulse of sufficient duration toallow alkylate signal means 56 to make the necessary calculations. Thepulse from multivibrator 60 triggers another one shot multivibrator 61which provides control pulse Ep.

Pulse Ep triggers yet another one shot multivibrator 62 to providecontrol pulse E A T, computer provides a signal E corresponding to acalculated temperature T, in accordance with signals 13, E and E directcurrent voltages V, through V pulse 1'1 and equation 20.

Referring now to FIG. 3, pulse E resets a counter in T, computer 70.Counter 75 is a conventional updown counter. Signal E from a controlcomputer 71 enables an AND gate 76 to pass timing pulses from clockmeans 78 to counter 75. Counter 75 counts the timing pulses passed byAND gate 76. The count in counter 75 is decoded by a logic decoder 82which provides a plurality of outputs corresponding to the count incounter 75. By way of example, one output from logic decoder 82 will beat a high level for a particular count in counter 75 and at a low levelwhen that count is not in counter 75. Thus when counter 75 reaches afirst predetermined count, an output from count 75 is applied to anelectronic switch 83 goes to a high level rendering switch 83 conductiveto passed voltage V;,. When the output from counter 75 is at a lowlevel, switch 83 is rendered non-conductive to block voltage V,,.Voltage V, corresponds to a value of 0.0 for the term j in equation 20.Similarly switches 83A, 83B and 83C receive direct current voltages V Vand V,

lnc., assignee of corresponding to the concentration of the com-.

7 corresponding to values 1.0, 2.0 and 3.0, respectively; for the term jin equation 20.

It should be noted that if there are more values desired for the term j,more switches need to be connected to logic decoder 82. It can bereadily seen that in effect counter 75 and logic decoder 82 controlswitches 83 through 83C so that the different voltages may be used inequation 20 for different values ofj to determine the T, temperature. Avoltage passed by a switch 83, 83A, 838 or 83C is applied to amultiplier 84 receiving voltage V to provide a signal, corresponding tothe term 2j, to a divider 85 which also receives voltage V whichcorresponds to the term J in this instance. Divider 85 provides a signalcorresponding to 2j/J. Subtracting means 87 subtracts voltage V, fromthe output from divider 85 to provide a signal to a mu]- tiplier 92corresponding to the term [(2j/J) l] in equation 20. Multiplier 92multiplies the signal from subtracting means 87 with voltage V,corresponding to a maximum allowable temperature change (A T The outputfrom multiplier 92 is summed with signal E by summing means 93 toprovide signal E Signal E corresponds to the temperature T of the acidhydrocarbon mixture leaving contactor 4 by a sensor 91 in line 8. V

The outputs from logic decoder 82 are inverted by plurality of inverters94 through 94C and are applied to corresponding time delay one shotmultivibrators 94 through 95C, respectively. When an output from decoder82 goes to a high level in response to a particular count in counter 75,an inverter 94, 94A, 948 or 94C inverts the output to a low levelthereby triggering a corresponding multivibrator 95, 95A, 95B or 95C,respectively. A pulse from the triggered one shot multivibrator 95, 95A,95B or 95C passes through an OR gate 96 and the trailing edge of thepassed pulse triggers another one shot multivibrator 99 causing it toprovide a sampling pulse E The pulse width of the pulses provided bymultivibrators 95 through 95C are of such a duration as to allow thevarious computers to make the calculations based on the new T,temperature before sampling and holding is performed.

In operation, signal E is at a high level unless a certain predeterminedcondition exists. For example, if the earnings decrease or theconstraint on an operating parameter is exceeded at the new calculatedoperating condition, then E goes to a low level. However, since the newcalculated operating condition is an undesirable condition, E remains ata high level for a period of time before changing to a low level toallow one more timing pulse from clock means 78 to pass through AND gate76. Counting direction signal E changes immediately from a high level toa low level so that the last pulse passed by AND gate 76 causes thecount in counter 75 to be reduced by one. The net result is that thecontrol system is returned to its next previous calculation step whichis the most desirable operating condition.

The T temperature signal E from sensor 91 is also applied to a Gcomputer 100, to an F computer 100A and to a H, computer 101 while T,temperature signal E from T, computer 70 is applied to a computer 1008,to an F computer 100C and to an H computer 101A. Computers 100A, 100Band 100C are similar to computer 100 except that computer 100A differsfrom computer 100 in receiving different direct current voltages so itcan solve equations l4, l and 16, while computer solves equations 4, 5and 6. Computers 100B, 100C differ from computer 100 and 100A,respectively, in that they use the T,- temperature instead of the sensedtemperature T,, in their calculations. Therefore, it is necessary onlyto explain the operation of computer 100.

Referring to FIG. 4, there is shown the G computer 100 whichincludes-computing circuit 105. Circuit 105 receives signal E and directcurrent voltages V through V,,, which correspond to the lower referencetemperature T,, the constant of 100 and the coefficients a, through arespectively, in equation 4. Computing circuit 105 includes subtractingmeans 106 which subtracts voltage V, from signal E to provide a signalto a divider 107 receiving voltage V,. Divider 107 provides a signalcorresponding to the term [(T,,-T1)/100]. The signal from divider 107 iseffectively squared by a multiplier 110 to provide a signalcorresponding to the [(T,,T )/l00] which is applied to series connectedmultipliers 111 through 113. Multipliers 111, 112 and 113 provideoutputs corresponding to the quantities [(T -T1)/l00] [(T,r,T,)/l00] andT T )/10O] respectively. A plurality of multipliers through 124 multiplythe outputs from divider 107 and multipliers 110 through 113,respectively, with voltages V through V respectively. The outputs frommultipliers 120 through 124 are summed by summing means 126 to provide asignal E corresponding to the term 6,, in equation 4.

Another computer circuit 105A receives signal E and direct currentvoltages V, and V through V Voltages V through V correspond to thecoefficients b, through 17;, in equation 5. Computer circuit 105Aoperates in a similar manner as computer circuit 105, except thatvoltages V and V through V replace voltages V and V through Vrespectively, to provide a signal E which corresponds to the term G inequation 5 for the condition that the propylene ratio is greater thanthe second predetermined reference ratio which by way of example may be0.60.

Subtracting means 130, 131 and 132, dividers 135 and 136, multipliers137 and 138 and summing means 139 cooperate to provide a signal Ecorresponding to the term G in equation 6 for a propylene ratio thatlies within the upper and lower propylene reference ratios in accordancewith equation 6, signals E E and E and direct current voltage V and Vfrom source 28. Voltages V and V correspond to the predetermined upperand lower propylene reference ratios, respectively. Subtracting means130 subtracts signal E, from voltage V to provide a signal correspondingto the term (P -P in equation 6, while subtracting means 131 subtractsvoltage V from signal E to provide a signal corresponding to the term (P-P Subtracting means 132 subtracts voltage V from voltage V to provide asignal corresponding to the term (P -P in equation 6. Divider 135divides the signal from subtracting means 130 with the signal fromsubtracting means 132 to provide a signal, corresponding to the term [(PP )/(P -P which is multiplied with signal E by multiplier 137. Theoutput from multiplier 137 corresponds to the term G,,[(P -P,)/- (P -P0]in equation 6. Similarly, divider 136 divides the signal fromsubtracting means 131 with the signal from subtracting means 132 toprovide a signal which is multiplied with signal E by multiplier 138.Multiplier 138 provides a signal corresponding to the term 9 6,, [(P,,P)/(P P to summing means 139. Summing means 139 sums the signals frommultipliers 137, 138 to provide signal E The proper G signal is selectedby comparing signal E with voltages V V The comparison is then used tocontrol switching means to pass the selected 6,, signal. The comparisonis made by comparators 145 and 145A, while the switching means includeinverters 146 and 146A, an AND gate 150 and electronic switches 153, 151and 152. Comparators 145, 145A compare signal E with voltages V and Vrespectively. When the propylene ratio signal E is more positive thanvoltages V and V comparators 145 and 145A provide a low level and a highlevel output, respectively, to inverters 146, 146A, respectively. Theoutputs from comparators 145, 145A are also provided to AND gate 150.The high output from comparator 145A is inverted to a low level byinverter 146A which disables electronic switch 151 so that switch 151blocks signal E Since the output from comparator 145 is at a low levelit is inverted to a high level by inverter 146 to render electronicswitch 152, conductive so that switch 152 passes E as the G signal E Thelow output from comparator 145 disables AND gate 150 so that AND gate150 provides a low output to render electronic switch 153 non-conductivethereby blocking signal E For the condition when the propylene ratio iswithin the upper and lower limits, comparators 145, 145A provide highlevel outputs which cause AND gate 150 to enable switch 153. Whenenabled switch 153 passes signal E as signal E Inverters 146, 146Ainvert the high level outputs from comparators 145 and 145A,respectively, to low level voltages which render switches 152 and 151non-conductive, respectively. Switches 151, 152 block signals E and Erespectively.

For the condition when the propylene ratio is less than the lower limit,comparators 145, 145A provide a high level and low level output,respectively. The high level output from comparator 145 is inverted byinverter 146 to render switch 152 non-conductive to block signal E Thelow level output from comparator 145A disables AND gate 150 causing itto render electronic switch 153 non-conductive thereby blocking signal EThe low level output from comparator 145A is inverted to a high level byinverter 146A which renders electronic switch 151 conductive to passsignal E as signal E Voltages V V V V and V are provided by source 28 tocomputers 100A, 1008 and 100C, while voltages V to V and V to V are alsoapplied to computer 1008, and voltages V through V V through V areapplied to computers 100A and 100C. In this regard, voltages V through Vcorrespond to the coefficients through 0 in equation 14, voltages Vthrough V correspond to the coefficients d, through d in equation 15,while voltage V corresponds to the term 1.0 in equations 14 and 15. Itshould be noted that the term 1.0 in equations 14 and 15 is not presentin equations 4 and 5. Computers 100A and 100C have computing circuitssimilar to computing circuit 105, which sum voltage V with the signalsfrom the multipliers as is done by summing means 126 with the outputsfrom multipliers 120 through 124 in computing circuit 105.

Referring to FIG. 5, signal E is provided to multipliers 160 through165. Multiplier 160 in 11,, computer 101 effectively squares signal E toprovide a signal corresponding to the term T in equation 18 tomultipliers 165 through 169. It should be noted that since l-l computer101 uses signal E the Tin equation 18, for 5 purpose of evaluation, maybe replaced by T Multipliers 161 through 164 multiply signal E withdirect current voltages V through V from source 28, which correspond tothe coefficients e e e and e in equation 18, to provide outputsrepresentative of the terms e T 2 T e T and 2 T respectively.Multipliers 166 through 169 multiply the output from multiplier 160 withdirect current voltages V through V from source 28 which correspond tothe coefficients e e e and e in equation 18 to provide outputsrepresentative of the terms e T e T e T and e, T respectively.

Multiplier 165 multiplies the output from multiplier 160 with signal Eto provide an output corresponding to the term T in equation 18.Multipliers 170 through 173 multiply the output from multiplier 165 withdirect current voltages V through V from source 28 which correspond tothe coefficients e e e and e respectively, in equation 18 to provideoutputs representative of the terms e T e T e T and e T Summing means177 sums the outputs of multipliers 161, 166 and 170 with a directcurrent voltage V from source 28 which corresponds to the coefficient e,to provide a signal corresponding to the term (e, e T e T e T Amultiplier 178 effectively squares signal E to provide a signalcorresponding to the term P while another multiplier 179 multiplies thesignal from multiplier 178 with signal E to provide a signalcorresponding to the term P Summing means 180 sums the outputs frommultipliers 162, 167 and 171 with a direct current voltage V from source28 corresponding to the coefficient e The signal from summing means 180is multiplied with signal E, by a multiplier 185 to provide a signalcorresponding to the term (e +e T +e T +e T )P in equation 18. Summingmeans 181 sums the outputs from multipliers 163, 168 and 172 with adirect current voltage V from source 28 corresponding to the coefficiente A multiplier 186 multiplies the signal from summing means 181 with theoutput from multiplier 178 to provide a signal corresponding to the term(e9+e oTB+ e T +e T )P in equation 18. Summing means 182 sums theoutputs from multipliers 164, 169 and 173 with a direct current voltageV from source 28 corresponding to the coefficient e A multiplier 187multiplies the signal from summing means 182 with the output frommultiplier 179 to provide a signal which corresponds to the term (e, +e,T +e T +e T )P in equation 18. Summing means 188 sums the output fromsumming means 177 and multipliers 185, 186 and 187 to provide signal Ecorresponding to H H computer 101A operates in a similar manner as Hcomputer 101 except that computer 101A uses the T, signal E instead ofthe T signal E Referring to FIG. 1, subtracting means 200 subtractssignal E provided by G computer 100 from signal E provided by G computer1008 to provide a signal E corresponding to the A Q term of equations 2and 3. The signal from subtracting means 200 is applied to controlcomputer 71.

A divider 201 divides signal E from F computer 100A by signal E from Fcomputer 100C. Subtracting means 202 subtracts direct current voltage Vfrom source 28 from the output from divider 201 to provide a signalcorresponding to the term [(F /F l] in equation 7. A multiplier 207multiplies the output from subtracting means 202 with a signal E whichcorresponds to the acid consumption A to provide a signal Ecorresponding to the term A A in equations 2 and 7, to control computer71.

Signal B is developed in the following manner. A multiplier 208multiplies the alkylate concentration signal E, from alkylate signalmeans 56 with a hydrocarbon flow rate signal E from a sensor 206 in line14, to provide a signal corresponding to the term R in accordance withequation l3. A flow rate sensor 236 senses the flow rate of thedischarge acid in line 21 and provides a signal E corresponding to thesensed discharge acid flow rate R,,,,. Signal E is divided into thesignal from multiplier 208 by a divider 209 to provide a signal toanother multiplier 210. Multiplier 210 multiplies a signal provided bydivider 209 with a direct voltage V which corresponds to the constant ain equation 8, from source 28 to provide signal E Subtracting means 215subtracts signal E provided by H computer 101 from signal E provided byH computer 101A. Subtracting means 215 provides an output Ecorresponding to AU in equations 2 and 17, to control computer 71.

Referring to FIGS. 1 and 6, control computer 71 controls the operationof the alkylation unit by determining four conditions: whether or notthe trial temperature T, exceeds predetermined temperature limits T andT whether or not the calculated utility rate R exceeds predeterminedutility rate limits R and R whether or not the calculated discharge acidflow rate R exceeds predetermined discharge acid flow rate limits R andR and whether or not the calculated earnings for the current calculationstep is greater than the earnings for the next previous calculationstep. Control computer 71 controls the alkylation unit in accordancewith Signals 11 14 22, 25 25 29 30 33 and 34 and direct current voltagesV through V from source 28.

Signal E corresponding to the temperature T,, is applied to comparators220, 221 receiving the upper temperature limit T voltage V and the lowertemperature limit T voltage V respectively. Comparators 220, 221 providea low level and a high level direct current output, respectively, whensignal E is more positive than voltages V V high level outputs whensignal E is more positive than voltage V but not more positive thanvoltage V and a high level and a low level output, respectively, whensignal E is not more positive than voltages V V An AND gate 224 providesa high level direct current output when comparators 220, 221 providehigh level outputs and a low level output when either comparators 220and 221, or both comparators, provide a low level output. Thus, AND gate224 provides a high level output when the calculated temperature T, doesnot exceed the predetermined limits and a low level output when T, doesexceed a predetermined limit.

Signal E corresponding to the term (A U in equations 2, l7 and 19provided by subtracting means 215 is multiplied with the alkylate rate Routput from multiplier 208 by a multiplier 228. The output provided bymultiplier 228 corresponds to the term (A U (R in equation 19. Summingmeans 229 sums a signal E corresponding to the coolant flow rate,provided by a sensor 230, with the output from multiplier 228 to providea signal E corresponding to the term R in equation 19.

Comparators 220A and 221A and an AND gate 224A cooperate in a mannersimilar to the cooperation of comparators 220, 221 and AND gate 224 tomake the determination as to whether or not the rate of the utilityexceeds predetermined limits. Comparators 220A, 221A compare signal Ewith direct current voltages V and V from source 28 corresponding to thepredetermined upper and lower limits, respectively, for the utility rateR and R respectively. AND gate 224A provides a high level output whenthe utility rate R does not exceed the predetermined limits and a lowlevel output when R exceeds a predetermined limit.

The calculated discharge acid flow rate is computed in accordance withequation 12. A multiplier 235 multiplies a signal E corresponding to thedischarge acid flow rate provided by a sensor 236 in line 21, withsignal E from divider 201 to provide a signal E corresponding to thecalculated discharge acid flow rate R in equation 12 for the currentcal-culation step.

Comparators 2208 and 221B cooperate with AND gate 2248, in a mannersimilar to comparators 220, 221 and AND gate 224, to determine whetherthe calculated discharge acid flow rate R is within predeterminedlimits. Comparators 2208 and 221B compare signal E with direct currentvoltages V and V corresponding to a predetermined upper flow rate limitR and to a predetermined lower flow rate limit R respectively, for thedischarge acid in line 21. AND gate 224B is controlled by comparators2208 and 2218 to provide a high level output when the calculateddischarge acid flow rate R does not exceed predetermined limits definedby voltages V V and a low level output when R does exceed apredetermined limit.

Multipliers 240, 241 and 242 and subtracting means 246 and 247 computethe earnings in accordance with equation 2, signals E E and E and directcurrent voltages V V and V from source 28. Multipliers 240, 241 and 2 42multiply signals E E and E respectively, with voltages V V and Vrespectively. Voltages V V and V correspond to the terms W W and Wrespectively. Multipliers 240, 241 and 242 provide outputs correspondingto the terms (AQ) (W (AA) (W and (AU) (W respectively, in equation 2.Subtracting means 246 subtracts the output provided by multiplier 241from the output provided by the multiplier 240 to provide an output tosubtracting means 247. Subtracting means 247 subtracts the outputprovided by multiplier 242 from the output provided by subtracting means246 to provide an output to an earnings comparison network 253.

Earnings comparison network 253 compares the earnings for the currentcalculation step with the earnings for the next previous calculationstep and includes sample and hold circuits 254, 255, a one-shotmultivibrator 256 and a comparator 257. The output from subtractingmeans 247 is applied to sample and hold circuit 254 while sampling pulseE from T, computer are applied to multivibrator 256 and to sample andhold circuit 255. One shot multivibrator 256 provides a pulse output tosample and hold circuit 254 in response to the trailing edge of a pulseE The sequence of sampling and holding is such that sample and holdcircuit 255 holds the output from sample and hold circuit 254. Theoutput provided by sample and hold circuit 255 corresponds to theearnings for the next previous calculation step while the output fromsample and hold circuit 254 corresponds to the earnings for the currentcalculation step. Comparator 257 compares the outputs from sample andhold circuits 254 and 255 and provides a high level direct currentoutput when the output from sample and hold circuit 254 is more positivethan the output from sample and hold circuit 255 and a low level outputwhen the output from sample and hold circuit 254 is not more positivethan the output from sample and hold circuit 255. Thus comparator 257provides a high level output when the current calculated earnings aregreater than the earnings for the next previous calculation step, andprovides a low level output when the current earnings are equal to orless than the next previous earnings. An AND gate 260 providesdirectional signal E at a high level when AND gates 224, 224A, 2243 andcomparator 257 provide high level outputs and a low level output whenone or more of the outputs from AND gates 224, 224A and 224B andcomparator 257 is at a low level.

Signal E from AND gate 260 is applied to an OR gate 261 and to aone-shot multivibrator 262. As each calculation satisfies all of theaforementioned conditions, signal E from AND gate 260 remains at a highlevel and passes through OR gate 261 to become signal E During thecourse of a calculation if one of the limits is exceeded or the earningsdoes not increase, signal E from AND gate 260 goes to a low level andtriggers one-shot multivibrator 262. It would be expected that signal Ewould go to a low level when signal E did. However, if this was tooccur, the control operation would remain in an undesirable state sothat it is necessary to go back one calculation step. This is providedfor by the pulse output from one-shot multivibrator 262 which passesthrough OR gate 261 to become signal E so that signal E remains at ahigh level for duration of the pulse from multivibrator 262 while signalE is at a low level. The duration of the pulse from one-shotmultivibrator 262 is such that it enables one more pulse E from clockmeans 78 to pass through AND gate 76. The low level signal E from ANDgate 260 causes the counter to count down the last pulse passed by ANDgate 76 resulting in all of the computers being returned to their nextprevious calculation step.

When the desired operating condition has been determined, controlcomputer 70 implements the required changes. The pulse frommultivibrator 262 which returns the control system to the next previouscalculation step, triggers a one-shot multivibrator 268 causing it toprovide a time delay pulse. The time delay is to allow time for all ofthe calculations of the next previous calculation step to be made. Thetrailing edge of the pulse from multivibrator 268 triggers another oneshot multivibrator 269 causing it to provide an enter" pulse E to setpoint signal means 270, 270A. Set point signal means 270 also receivessignals E E and provides a directional signal E and a pulse signal E. toa temperature recorder controller 271. Signals E E position the setpoint ofcontroller 271 so as to control the temperature of contactor 4.Subtracting means 273 in signal means 270 subtracts signal E from signalE to provide an output corresponding to the change in the contactortemperature required to achieve the desired operating condition. Acomparator 277 compares the output from subtracting means 273 with aground reference 278 to provide directional signal E When the selectedtemperature, which is now temperature T is less than the sensedtemperature T the output from subtracting means 273 is negative andhence is less positive than ground 278 causing comparator 277 to providea low level direct current output as directional signal E When thesensed temperature T is less than the selected temperature T,-, theoutput from subtracting means 273 is positive causing comparator 277 toprovide a high level output as signal E A conventional type analog-todigital converter 276 converts the output from subtracting means 273 todigital signals which are applied to a count down counter 280. The entryof digital signals into counter 280 is controlled by enter pulse E frommultivibrator 269 and presets counter 280 to the desired change intemperature.

The trailing edge of the enter pulse E from multivibrator 269 triggers amultivibrator 286 which provides an enabling pulse to an AND gate 287.AND gate 287 is connected to counter 280 in such a manner that counter280 provides a low level direct current signal to AND gate 287 when thecount in counter 280 is zero and a high level direct current signal whenthe count in counter 280 is not zero. Clock means 290 provides timingpulses to AND gate 287. As a result of a count in counter 280 not beingzero and the enabling pulse from multivibrator 286, and timing pulsesfrom clock means 290 pass through AND gate 287 to be counted down bycounter 280 from the entered count. The timing pulses passed by AND gate287 are also provided as pulse signal E to temperature recordercontroller 271. Each pulse in pulse signal E changes the position of theset point in temperature recorder controller 271 by a predeterminedamount and in a direction in accordance with signal E The set pointchanging process continues until a count of zero is reached by counter280, at which time counter 280 output to AND gate 287 goes to a lowlevel thereby disabling AND gate 287. When disabled, AND gate 287 blocksthe timing pulses from clock means 290. It is obvious that the durationof the enabling pulse from multivibrator 286 must be such to allow thelargest anticipated count in counter 280 to be counted down to zero.

Temperature recorder controller 271 receives signal E from thetemperature sensor 91 in line 8 and provides a signal to a valve 295which controls the flow rate of the coolant flowing through coil 13.Temperature recorder controller 271 controls valve 295 in accordancewith signal E and the position of its set point so that the temperatureof the acid-hydrocarbon mixture leaving contactor 4 will assume thedesired temperature as represented by the position of temperaturerecorder controller 271 set point.

Set point signal means 270A is similar in construction and operation toset point signal means 270 except that the sensed discharge acid flowrate signal E replaces signal E and the calculated discharge acid flowrate signal E replaces signal E Set point signal means 270A provides adirectional signal E and a pulse signal E instead ofdirectional signal Eand pulse signal E respectively, to a flow recorder controller 300 whichalso receives signal E Flow recorder controller 300 provides a signal toa valve 301 in line 21 which controls the flow rate of the dischargeacid. Set point signal means 270A is activated by enter pulse E fromone-shot multivibrator 269 to provide signals E E In response to signalsE and E the set point of flow recorder controller 300 changesaccordingly and provides a signal to valve 301 which corresponds to thedifference between the sensed signal E and the position of set pointcausing valve 301 to increase or decrease the discharge acid flow ratein accordance with the position of the set point of flow recordercontroller 300 so that the discharge acid flow rate is the desireddischarge acid flow rate.

Since the acid strength changes when the contact temperature is changed,the discharge acid flow rate is changed to maintain the strength of theacid entering contactor 4. In this regard, when the discharge acid flowrate is increased, the acid level in settler 12 would decrease. A levelsensor 303 provides a signal to a level controller 304. Level controller304 provides a signal which corresponds to the difference between thesensed level and a predetermined level, to a valve 308 in line 17 inaccordance with the signal from sensor 303 and the position oflevelcontroller 304 set point. The signal from level controller 304causes valve 308 to increase the fresh acid flow rate so as to increasethe acid level in settler 12. The increase in the fresh acid flow rateincreases the strength of the acid entering contactor 4. Similarly, adecrease in the discharge acid flow rate will result in a decreased acidstrength.

The system of the present invention as heretofore described controls analkylation unit to achieve an optimum operating condition for thealkylation unit. The discharge acid, the contactors temperature and thealkylate product are monitored to determine values using equationsheretofore described so that earnings may be determined. The controlsystem uses a set of trial contactor temperatures and calculates theeffect of a change in contactors temperature on the earnings. Thecontrol system of the present invention as heretofore described controlsan alkylation unit to achieve the optimum operating condition withoutexceeding a constraint value of an operating parameter.

What is claimed is:

l. A system for controlling an alkylation unit to achieve an optimumoperating condition and said alkylation unit includes a contactorwherein an olefinisoparaffin mixture is contacted with acid at atemperature controlled by utility means in accordance with a controlsignal and the contactor provides an acidhydrocarbon mixture to asettler which separates the acid to provide a hydrocarbon product whichinclude alkylate and acid, a portion of the separated acid is dischargedwhile a portion of the separated acid is fed back to the contactor alongwith fresh acid entering the alkylation unit that is added to thefeedback to replace the discharge acid, comprising means for sensing thecontact temperature T and providing a signal corresponding thereto,means for sensing a utility rate and providing a signal correspondingthereto, means for sensing a condition related to the contact acid andproviding a corresponding signal, means for sensing conditions of thehydrocarbon product and providing signals representative thereof, meansfor sensing the olefin and providing a signal representative thereof,means for providing signals corresponding to economic values W W and Wrelated to the octane rating of the alkylate, to the acid consumptionand to the utility, respectively, means for controlling the acidentering or leaving the alkylation unit in'accordance with a controlsignal and means connected to all of the other means for providing thecontrol signals to the acid control means and to the utility means inaccordance with the signals from the sensing means and from the signalmeans so as to achieve a desired operating condition for the alkylationunit.

2. A system as described in claim 1 in which the utility means includesmeans for passing a coolant through the contactor so as to control thecontact temperature.

3. A system as described in claim 2 in which the control signal meansdetermines the effect that a change in the contact temperature wouldhave on the earnings of the alkylation unit.

4. A system as described in claim 3 in which the control signal meansincludes first means connected to the temperature sensing means, to theolefin sensing means and to the hydrocarbon product sensing means forcalculating values of factors G F and H from the signals from thetemperature sensing means, from the olefin sensing means and from thehydrocarbon product sensing means, means connected to the temperaturesensing means for providing a signal, corresponding to a trialtemperature T in accordance with the sensed contact temperature T signaland the following equation:

1. T, T [(2j/J l A T where T is the sensed contacting temperature, J isany positive integer and j is iterativeely changed in value between 0and J; in response to a command signal, second means connected to thetrial temperature signal means, to the hydrocarbon product sensing meansand to the olefin sensing means for calculating values of factors G Fand H in accordance with a trial temperature signal from the trialtemperature signal means and the signals from the hydrocarbon productsensing means and from the olefin sensing means and providing signalscorresponding thereto; means connected to the acid sensing means, to thehydrocarbon product sensing means and to the first and secondcalculating means for determining changes A Q, A A and A U in the octanerating, the acid consumption and the utility rate, respectively, meansconnected to the change determining means and to the economic valuesignal means for determining the earnings for the alkylation unit for acurrent trial temperature and providing an earnings signal correspondingthereto for earnings control means connected to the earnings signalmeans and to the trial temperature signal means for providing a commandsignal to the trial temperature means causing the trial temperaturemeans to iteratively change the trial temperature until the commandsignal is representative of a decrease in earnings and then to controlthe trial temperature signal means to the next previous trialtemperature, and means connected to the earnings control means, to theutility means, to the acid control means, means for providing thecontrol signals to the utility means and to the acid control means inresponse to the command signal from the earnings control means beingrepresentative of a decrease in earnings.

5. A system as described in claim 4 in which the olefin sensing meansincludes means for sensing concentrations of the different olefins andproviding signals corresponding thereto, means connected to the olefinconcentration sensing means for providing a signal corresponding to theratio P of propylene to olefins; and the first and second calculatingmeans includes G signal means connected to the temperaturesensing meansand to the trial temperature signal means, respectively, and receivingdirect current voltages corresponding to reference temperatures T and Tand predetermined coefficients a through a and b through b for providingsignals corresponding to the quantities G and G in accordance with thefollowing equations:-

GL a1 a2 a 1/100l 4 l( 1)/ 1)/ u 1 2)/100] 2 l( 2)/ l 3 [(T-TQ/IOOP, andb, [(TT )/100] b (TT2)/lO0] 161! [(PU PB)/(PU PL)] 1. [(PB PL)/(PUPL)] Uso that a signal G or G is provided in accordance with each equation byeach G signal means using sensed temperature T and the trial temperatureT, for the term T to provide the G signal, first switchingmeans'receiving direct current voltages corresponding to referenceratios P and P and connected to the P ratio signal means and to each Gsignal means for selecting the proper signal G or G in accordance withthe ratio P signal and the P and P reference voltages so that thesignals corresponding to the term G L are provided as the G and Gsignals to the change determining means when the ratio P is morenegative than the lower reference ratio P signal, and signalscorresponding to the term G,, are provided as the G and G signals to thechange determining means when the ratio signal P is more positive thanthe upper reference ratio P voltages, the signals corresponding to theterm G are provided as the G and G signals to the determining means whenthe sensed ratio P signal is not more negative than the lower referenceratio P voltage nor more positive than the upper reference ratio Pvoltage; F signal means connected to the temperature sensing means andto the trial temperature signal means and receiving the direct currentvoltages corresponding to reference temperatures T and T a term of 1.0and predetermined coefficients c through 0 and 11 through (i forproviding signals corresponding to quantities F and F in accordance withthe following equations:

F 1.0 c [(T--T )/lOO] 0 [(TT )/l0O] 3[( l)/ F 1.0 d [(T-T )/l00] d [(T-T100] d ,[(T--T )/l00-rr 4]( 2)/ 5[( 2)/ L-ll y n)/( u U] FL B PL)/(P U]u second switching means receiving the direct current voltagescorresponding to the reference ratios P and P and connected to the Pratio signal means and to each F signal means for selecting the propersignal F or F in accordance with the P signal and the P and P referencevoltages sov that signals corresponding to the term F L are provided asthe F and F signals to the change determining means when the ratio P ismore negative than the P reference voltage, signals corresponding to theterm F are provided as the F and F signals to the change determiningmeans when ratio P signal is more positive than reference ratio Pvoltage, and signals corresponding to the term F are provided as the Fand F signals to the change determining means when the ratio P signal isnot more negative for the term T to provide the G signal,

than the reference ratio P voltage; and means connected to thetemperature sensing means, to the trial temperature signal means and tothe ratio P signal means and receiving direct current voltagescorresponding to the coefficients e through e for providing signalscorresponding to the quantities H and H in accordance with the followingequation:

l3 l4 15 l6 H where the sensed temperature T is used for T to providethe signal H and the trial temperature T, is used for T to provide thesignal corresponding to the calculated quantity H 6. A system asdescribed in claim 4 in which the hydrocarbon product sensing meansincludes means for sensing the flow rate R of the hydrocarbon productand providing a signal corresponding thereto, means for sensing thealkylate content V of the hydrocarbon product and providing acorresponding signal, and means connected to the hydrocarbon productflow rate sensing means and to the alkylate content sensing means forproviding a signal corresponding to the alkylate flow rate R inaccordance with the following equation:

RK VKRC and the acid sensing means includes means for sensing the flowrate R of discharge acid and providing a signal corresponding thereto.

7 A system as described in claim 6 in which the change determining meansincludes means connected to the earnings signal means and to the firstswitching means for subtracting the G signal from the G signal toprovide a signal to the earnings signal means, corresponding to thechange in octane rate A Q, means connected to the alkylate flow ratesignal means and to the discharge acid flow rate sensing means andreceiving a direct current voltage corresponding to a predeterminedconversion factor a for providing a signal corresponding to the acidconsumption A B by the alkylation unit inv accordance with the followingequation:

AB os/ x means receiving a direct current voltage corresponding to aterm 1 in the next following equation and connected to the secondswitching means to the earnings signal means and to the acid consumptionsignal means for providing a signal corresponding to the differentialacid consumption A A to the earnings signal means in accordance with thefollowing equation:

A AB[(FB/FC)"1] and means connected to the H and H signal means and tothe earnings signal means for subtracting the H signal from the H signalto provide a signal to the earnings signal means corresponding to thechange A U in the utility rate.

8. A system as described in claim 7 in which the earnings signal meansprovides the earnings signal in accordance with the A Q,AA,AU,W W and Wsignals and the following equation:

Q)( a) AXWA) U v) where E is the earnings; and the earnings controlmeans includes means connected to the earnings signal means for samplingand holding the earnings signal to provide a pair of signals, one signalcorresponding to the earnings for a current trial temperature while theother signal corresponds to the earnings for a next previous trialtemperature, comparing means connected to the sample and hold means andto the trial temperature signal means and responsive to the signals fromthe sample and hold means for providing a high level direct currentsignal as the command. signal to the trial temperature signal means whenthe earnings for the current trial temperature is not less than theearnings for the next previous trial temperature and providing a lowlevel direct current signal as the command signal to the trialtemperature signal means when the earnings for the current trialtemperature is less than the earnings for the next previous trialsignal, and temperature control signal means connected to the firstcomparing means, to the trial temperature means and to the utility meansfor providing the signal corresponding to the next previous trialtemperature as the temperature control signal in response to the commandsignal from the comparing means changing from a high level to a lowlevel.

9. A system as described in claim 8 in which the earnings control meansfurther comprises means connected to the AU signal means, to thedischarge acid flow rate sensing means and to the alkylate flow ratesignal means for providing a signal corresponding to a utility rate R inaccordance with the A U signal, the R signal and the R K signal and thefollowing equation:

u os X K), and means connected to the acid consumption A signal means,to the A A signal means and to the alkylate flow rate R signal means andreceiving the direct current voltage corresponding to the conversionfactor a for providing a signal corresponding to a calculated dischargeacid flow rate R DC in accordance with the A A, A and R K signals,thedirect current voltage and the following equation:

(AB A x/ and the comparing means is also connected to the R signal meansandreceiving direct current reference voltages corresponding topredetermined limits for the trial temperature T for the utility rate Rand for the calculated discharge acid flow rate R and provides the highlevel signal to the temperature control signal means when the earningsfor the current trial temperature is not less than the earnings for thenext previous trial temperature, the trial temperature T, does notexceed a predetermined limit, the utility rate R does not the earningsfor the next previous trial temperature, the

trial temperature T,- exceeds a predetermined limit, the utility rate Rexceeds a predetermined limit, or the calculated discharge acid flowrate exceeds a predetermined limit.

10. A method for controlling an alkylation unit to achieve an optimumoperating condition and said alkylation unit includes a contactorwherein an olefinisoparaffin mixture is contacted with acid at atemperature controlled by utility means and the contactor provides anacid-hydrocarbon mixture to a settler which separates the acid from theacid-hydrocarbon mixture to provide a hydrocarbon product, whichincludes alkylate and acid, a portion of the separated acid isdischarged while a portion of the separated acid is fed back to thecontactor along with fresh acid entering the alkylation unit that isadded to feedback acid to replace the discharge acid, which comprisesthe following steps: sensing the contact temperature T sensing a utilityrate, sensing a condition related to the contact acid, sensingconditions to the hydrocarbon product, sensing conditions of the olefin,determining economic values related to the octane rating of thealkylate, to acid consumption and to the utility and controlling theacid entering the alkylation unit and the contact temperature inaccordance with the sensed contact temperature, the sensed utility rate,the sensed condition related to the contact acid, the sensed conditionof the hydrocarbon product and the sensed olefin conditions and thedetermined economic values, so as to achieve an optimum operatingcondition for the alkylation unit.

11. A method as described in claim 10 in which the contact temperatureis controlled by passing a coolant through the contactor.

12. A method as described in claim 11 in which the controlling stepincludes determining the effect that a change in the contact temperaturewould have on the earnings of the alkylation unit.

2. A system as described in claim 1 in which the utility means includesmeans for passing a coolant through the contactor so as to control thecontact temperature.
 3. A system as described in claim 2 in which thecontrol signal means determines the effect that a change in the contacttemperature would have on the earnings of the alkylation unit.
 4. Asystem as described in claim 3 in which the control signal meansincludes first means connected to the temperature sensing means, to theolefin sensing means and to the hydrocarbon product sensing means forcalculating values of factors GB, FB and HB from the signals from thetemperature sensing means, from the olefin sensing means and from thehydrocarbon product sensing means, means connected to the temperaturesensing means for providing a signal, corresponding to a trialtemperature Tj, in accordance with the sensed contact temperature TBsignal and the following equation:
 5. A system as described in claim 4in which the olefin sensing means includes means for sensingconcentrations of the different olefins and providing signalscorresponding thereto, means connected to the olefin concentrationsensing means for providing a signal corresponding to the ratio PB ofpropylene to olefins; and the first and second calculating meansincludes G signal means connected to the temperature sensing means andto the trial temperature signal means, respectively, and receivingdirect current voltages corresponding to reference temperatures T1 andT2 and predetermined coefficients a1 through a5 and b1 through b5 forproviding signals corresponding to the quantities GB and GC inaccordance with the following equations: GL a1 ((T-T1/100) + a2((T-T1/100)2 + a3 ((T-T1/100)3 + a4 ((T-T1)/100)4 + a5 ((T-T1)/100)5 GUb1 ((T-T2)/100) + b2 ((T-T2)/100)2 + b3 ((T-T2)/100)3, and + b4((T-T2)/100)4 + b5(T-T2)/100)5 GL U ((PU-PB)/(PU-PL))GL +((PB-PL)/(PU-PL)) GU so that a signal GB or GC is provided in accordancewith each equation by each G signal means using sensed temperature TBfor the term T to provide the GB signal and the trial temperature Tj forthe term T to provide the GC signal, first switching means receivingdirect current voltages corresponding to reference ratios PU and PL andconnected to the PB ratio signal means and to each G signal means forselecting the proper signal GB or GC in accordance with the ratio PBsignal and the PU and PL reference voltages so that the signalscorresponding to the term GL are provided as the GB and GC signals tothe change determining means when the ratio PB is more negative than thelower reference ratio PL signal, and signals corresponding to the termGU are provided as the GB and GC signals to the change determining meanswhen the ratio signal PB is more positive than the upper reference ratioPU voltages, the signals corresponding to the term GU L are provided asthe GB and GC signals to the determining means when the sensed ratio PBsignal is not more negative than the lower reference ratio PL voltagenor more positive than the upper reference ratio PU voltage; F signalmeans connected to the temperature sensing means and to the trialtemperature signal means and receiving the direct current voltagescorresponding to reference temperatures T1 and T2, a term of 1.0 andpredetermined coefficients c1 through c5 and d1 through d5 for providingsignals corresponding to quantities FB and FC in accordance with thefollowing equations: FL 1.0 + c1((T-T1)/100) + c2 ((T-T1)/100)2 +c3((T-T1)/100)3 + c4((T-T1)/100) + c5((T-T1/100)5 FU 1.0 +d1((T-T2)/100) + d2((T-T2)/100)2 + d3((T-T2)/100 pi 3 + d4)(T-T2)/100) +d5((T-T2)/100)5 FL U ((PU - PB)/(PU - PL)) FL + ((PB - PL)/(P - PL)) FUsecond switching means receiving the direct current voltagescorresponding to the reference ratios PU and PL and connected to the PBratio signal means and to each F signal means for selecting the propersignal FB or FC in accordance with the PB signal and the PU and PLreference voltages so that signals corresponding to the term FL areprovided as the FB and FC signals to the change determining means whenthe ratio PB is more negative than the PU reference voltage, signalscorresponding to the term FU are provided as the FB and FC signals tothe change determining means when ratio PB signal is more positive thanreference ratio PU voltage, and signals corresponding to the term FU Lare provided as the FB and FC signals to the change determining meanswhen the ratio PB signal is not more negative than the reference ratioPU voltage; and means connected to the temperature sensing means, to thetrial temperature signal means and to the ratio PB signal means andreceiving direct current voltages corresponding to the coefficients e1through e16 for providing signals corresponding to the quantities HB andHC in accordance with the following equation: H e1 + e2 T + e3 T2 + e4T3 + (e5+e6T+e7T2+e8T3)PB + (e9+e10T+e11T2+e12T3)PB2 +(e13+e14T+e15T2+e16T3)PB3 where the sensed temperature TB is used for Tto provide the signal HB and the trial temperature Tj is used for T toprovide the signal corresponding to the calculated quantity HC.
 6. Asystem as described in claim 4 in which the hydrocarbon product sensingmeans includes means for sensing the flow rate RC of the hydrocarbonproduct and providing a signal corresponding thereto, means for sensingthe alkylate content VK of the hydrocarbon product and providing acorresponding signal, and means connected to the hydrocarbon productflow rate sensing means and to the alkylate content sensing means forproviding a signal corresponding to the alkylate flow rate RK inaccordance with the following equation: RK VKRC and the acid sensingmeans includes means for sensing the flow rate RDB of discharge acid andproviding a signal corresponding thereto.
 7. A system as described inclaim 6 in which the change determining means includes means connectedto the earnings signal means and to the first switching means forsubtracting the GB signal from the GC signal to provide a signal to theearnings signal means, corresponding to the change in octane rate DeltaQ, means cOnnected to the alkylate flow rate signal means and to thedischarge acid flow rate sensing means and receiving a direct currentvoltage corresponding to a predetermined conversion factor Alpha forproviding a signal corresponding to the acid consumption AB by thealkylation unit in accordance with the following equation: AB AlphaRDB/RK means receiving a direct current voltage corresponding to a term1 in the next following equation and connected to the second switchingmeans to the earnings signal means and to the acid consumption signalmeans for providing a signal corresponding to the differential acidconsumption Delta A to the earnings signal means in accordance with thefollowing equation: Delta A AB((FB/FC) - 1) and means connected to theHB and HC signal means and to the earnings signal means for subtractingthe HB signal from the HC signal to provide a signal to the earningssignal means corresponding to the change Delta U in the utility rate. 8.A system as described in claim 7 in which the earnings signal meansprovides the earnings signal in accordance with the Delta Q, Delta A,Delta U,WQ, WA and WU signals and the following equation: E ( DeltaQ)(WQ) - ( Delta A)(WA) - ( Delta U )(WU ) where E is the earnings; andthe earnings control means includes means connected to the earningssignal means for sampling and holding the earnings signal to provide apair of signals, one signal corresponding to the earnings for a currenttrial temperature while the other signal corresponds to the earnings fora next previous trial temperature, comparing means connected to thesample and hold means and to the trial temperature signal means andresponsive to the signals from the sample and hold means for providing ahigh level direct current signal as the command signal to the trialtemperature signal means when the earnings for the current trialtemperature is not less than the earnings for the next previous trialtemperature and providing a low level direct current signal as thecommand signal to the trial temperature signal means when the earningsfor the current trial temperature is less than the earnings for the nextprevious trial signal, and temperature control signal means connected tothe first comparing means, to the trial temperature means and to theutility means for providing the signal corresponding to the nextprevious trial temperature as the temperature control signal in responseto the command signal from the comparing means changing from a highlevel to a low level.
 9. A system as described in claim 8 in which theearnings control means further comprises means connected to the Delta Usignal means, to the discharge acid flow rate sensing means and to thealkylate flow rate signal means for providing a signal corresponding toa utility rate RU in accordance with the Delta U signal, the RDB signaland the RK signal and the following equation: RU RDB + ( Delta U)(RK),and means connected to the acid consumption AB signal means, to theDelta A signal means and to the alkylate flow rate RK signal means andreceiving the direct current voltage corresponding to the conversionfactor Alpha for providing a signal corresponding to a calculateddischarge acid flow rate RDC in accordance with the Delta A, AB and RKsignals, the direct current voltage and the following equation: RDC(AB + Delta A)RK/ Alpha , and the comparing means is also connected tothe RU signal means and receiving direct current reference voltagescorresponding to prEdetermined limits for the trial temperature Tj, forthe utility rate RU and for the calculated discharge acid flow rate RDCand provides the high level signal to the temperature control signalmeans when the earnings for the current trial temperature is not lessthan the earnings for the next previous trial temperature, the trialtemperature Tj does not exceed a predetermined limit, the utility rateRU does not exceed a predetermined limit, and the calculated dischargeacid flow rate RDC does not exceed a predetermined limit, and providingthe low level signal when the earnings for the current trial temperatureis less than the earnings for the next previous trial temperature, thetrial temperature Tj exceeds a predetermined limit, the utility rate RUexceeds a predetermined limit, or the calculated discharge acid flowrate exceeds a predetermined limit.
 10. A method for controlling analkylation unit to achieve an optimum operating condition and saidalkylation unit includes a contactor wherein an olefinisoparaffinmixture is contacted with acid at a temperature controlled by utilitymeans and the contactor provides an acid-hydrocarbon mixture to asettler which separates the acid from the acid-hydrocarbon mixture toprovide a hydrocarbon product, which includes alkylate and acid, aportion of the separated acid is discharged while a portion of theseparated acid is fed back to the contactor along with fresh acidentering the alkylation unit that is added to feedback acid to replacethe discharge acid, which comprises the following steps: sensing thecontact temperature TB, sensing a utility rate, sensing a conditionrelated to the contact acid, sensing conditions to the hydrocarbonproduct, sensing conditions of the olefin, determining economic valuesrelated to the octane rating of the alkylate, to acid consumption and tothe utility and controlling the acid entering the alkylation unit andthe contact temperature in accordance with the sensed contacttemperature, the sensed utility rate, the sensed condition related tothe contact acid, the sensed condition of the hydrocarbon product andthe sensed olefin conditions and the determined economic values, so asto achieve an optimum operating condition for the alkylation unit.
 11. Amethod as described in claim 10 in which the contact temperature iscontrolled by passing a coolant through the contactor.
 12. A method asdescribed in claim 11 in which the controlling step includes determiningthe effect that a change in the contact temperature would have on theearnings of the alkylation unit.